Color silver halide photographic elements are generally processed by the steps of development, desilvering and washing and/or stabilization. In the color development step, the exposed silver halide is reduced with a color developing agent to generate silver whereupon the oxidized color developing agent reacts with a color coupler to provide a dye image. It is then necessary to remove the silver image which is formed coincident with the dye image. This can be done by oxidizing the silver using an oxidizing agent, known as a bleaching agent, in the presence of halide ion to form silver halide. The silver halide is then dissolved using a a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and fixing agent can be combined in a bleach-fixing solution and the silver removed in one step.
The above described desilvering process can be carried out in either a continuous or batch mode of processing. In the batch mode, all or part of the processing solutions are replaced whenever the amount of film processed reaches a predetermined quantity. In the continues mode, replenisher solutions are added to the processing solutions in an amount proportional to the amount of film processed. Such replenishment is generally expressed in milliliters per square foot or milliliters per number of rolls of film processed.
The pH of the developer must be alkaline in order for proper development to occur. In contrast, the pH of the bleach must by acidic. In some processing systems there is a stop bath in between the developer and the bleach which serves to modify the alkalinity of the developer. However, many modern bleaches act as both a stop bath and a bleach for metallic silver. It is therefore necessary to use bleach replenishers which have a lower pH then the bleach tank solutions into which they are replenished. This is done in order to offset the alkaline developer solution which is carried over into the bleach solution by the photographic element. Thus, the bleaching tank solution is generally of higher pH than the bleach replenisher solution.
To start up either a batch or replenished system it is necessary to make a bleach tank from a bleach replenisher solution. Bleach replenisher solutions are insufficient to provide desired photographic performance. When starting bleach tanks are prepared, a solution commonly known in the photographic industry as a "bleach starter" is added to the bleach replenisher solution. Water may also by added. The purpose of the bleach starter is to increase the pH of the bleach replenisher to the desired pH of the starting bleach tank solution.
Typically bleach starters are alkaline. Known bleach starters include ammonia, ammonium hydroxide, potassium hydroxide, potassium carbonate, and sodium hydroxide. U.S. Pat. Nos. 5,176,988 and 5,070,004 describe a bleach starter that contains an imidazole or a primary or secondary amine having a hydroxyalkyl radical as an alkaline agent. U.S. Pat. No. 5,217,852 discusses the use of many common alkali agents that may be used as bleach starters including potassium carbonate, ammonia water, imidazole, monoethanolamine or diethanolamine. U.S. Pat. Nos. 5,215,872, and 5,178,993 describe adjusting solutions which have bleaching ability with various alkali agents including potassium hydroxide, sodium hydroxide, imidazole, monoethanolamine, diethanolamine, imidazole, and most preferably aqueous ammonia.
Although all of the above mentioned alkaline agents are able to adequately adjust the pH of the bleach replenisher solution to that of a starting bleach tank, they all are undesirable due to odor or safety problems. Ammonia water, ammonium hydroxide and many amines cause an unpleasant odor themselves, or when mixed with the acidic replenisher solution. Other alkali agents, such as potassium hydroxide, cause undesirable precipitation when mixed with bleach replenishers, while agents such as potassium carbonate cause unwanted effervescence or foaming.
Thus, there is a need for a bleach starter that effectively adjusts the pH of a bleach replenisher solution without concomitant odor, precipitation, effervescence or safety problems.